Highly resistive gallium nitride (GaN) is an essential material for power optoelectronic applications. While carbon doping is widely used to achieve semi-insulating properties in GaN, the persistent photoconductivity (PPC) arising from deep-level defect traps remains a major obstacle for high-speed power switching. This study demonstrates a novel approach: leveraging the ultrahigh photoresponsivity of GaN:C (up to 2.1 A & centerdot;cm/W & centerdot;kV, surpassing alternatives such as GaN:Fe) and employing defect-selective optical control to effectively quench the PPC. By synchronizing a short infrared (1064 nm) quenching pulse with UV (385 nm) excitation in an epitaxially grown GaN:C layer on a heavily doped n-type GaN substrate, we achieve a dramatic reduction in photocurrent fall time by approximately 293 & times; (from 470 to 1.6 mu s), increasing modulation bandwidth from 745 Hz to nearly 218 kHz. This advancement not only establishes a new pathway for controlling PPC in GaN:C but also enables the practical integration of GaN:C in fast power switching devices. Enhanced modulation bandwidth, along with GaN:C excellent photoresponsivity, makes it a promising candidate for optically controlled high-voltage, high-power electronic systems, such as photoconductive semiconductor switches (PCSSs) used in pulsed-power drivers, high-power microwave (HPM) sources, and high-voltage gate drivers for wide bandgap (WBG) power electronics.

Semiconductor opening switches are solid-state devices capable of delivering nanosecond, hundreds of kilovolts pulses by interrupting kiloamps of current. The interruption of the current occurs in a moderately doped p-region when a high electric field region (HFR) is formed. The HFR occurs because the reverse pumping current cannot be supported by the saturation velocity and majority carrier concentration of the doping level. However, the donor profile also significantly affects the pulse performance. A secondary prepulse occurs if a secondary HFR is formed at the interface of the background n-doping and N+ doping (X-n). By moving the location of X-n deeper into the diode, the effect of the prepulse is reduced. This article investigates the effect of the donor doping profile on the performance metrics of semiconductor opening switches through technology computer-aided design (TCAD) simulations and experimental results. Through a SILVACO TCAD optimization, we designed a P+/p/n-base/n/N+ where the intersection of the moderate p-region and intrinsic n-base region (X-p) is at 160 mu m and X-n is at 220 mu m. This profile is fabricated via silicon epitaxy. Experimentally, it is shown that a deep X-n (220 mu m) compared with a shallow X-n (300 mu m) reduces the rise time by >5x. In addition, the magnitude of current density during interruption affects the prepulse foot and pulse shape. At lower current densities without the graded donor profile, high peak voltages are not achieved. Comparing the experimental results to the TCAD simulations shows that the model is predictive under high-current densities in the semiconductor opening switch (SOS) regime.

One of the primary advantages of β-Ga2O3 over incumbent wide bandgap semiconductors is the ability to grow directly from the melt. Melt growth, using Czochralski or similar methods, results in impurities in the crystal which originate from the crucible, such as iridium and other transition metals like chromium. These impurities exhibit optoelectronic signatures useful for their identification and sensitive to the Fermi energy of a given crystal (i.e., signatures vary with the electrical conductivity of the matrix). In this work, we describe how laser Raman systems can be used to map and spatially correlate Cr3+ photoluminescence, electronic-coupled Raman scattering from Ir4+d–d internal transitions, and the Raman line attributed to hydrogenic shallow donors. Laser ablation inductively coupled plasma mass spectrometry directly measured spatially dependent relative metal concentrations and confirmed spectroscopic signals resulting from heterogeneities in impurity concentrations in β-Ga2O3 boules. Mapping of photoluminescence and Raman-related signatures is, thus, demonstrated as an effective and facile method for spatial measurement of chemical heterogeneities in both insulating and conductive melt-grown β-Ga2O3 crystals.

Ultra-Wide Bandgap semiconductor transistors are promising to replace the current silicon-based devices for high-voltage, and high-power AC/DC conversion applications. Due to diamond’s superior physical properties, we have developed Diamond Optically Gated Field Effect Transistors (DOG-FET) with optimized Gate-All-Around (GAA) structure for fast switching, low jitter and low susceptibility to electromagnetic interference. Unlike the conventional GAA transistors, GAA in DOG-FET is defined by an optical aperture on top of the device. A three-dimensional gate around the conduction channel is formed by free electrons excited from deep nitrogen sites by the below bandgap light illumination. The gate position and width are directly controlled by the optical mask which allows for arbitrary gate distribution across the entire chip. The conduction channel is locked and memorized by shutting off the light. We have experimentally demonstrated optical gate control as well as channel memory effect which allows for device operation with pulsed light and pulsed voltage in planar devices. With the implementation of GAA, simulations suggest the devices are capable of switching Megawatts power at hundreds of kHz with <10% of light and voltage duty cycle.

Photoconductive semiconductor switching (PCSS) devices have unique characteristics to address the growing need for electrically isolated, optically gated, picosecond-scale jitter devices capable of operating at high voltage, current, and frequency. The state of the art in material selection, doping, triggering, and system integration in PCSSs is presented. The material properties and doping considerations of GaN, GaAs, SiC, diamond, and β-Ga2O3 in the fabrication of PCSS devices are discussed. A review of the current understanding of the physics of the high-gain mode known as lock-on is presented.

Monoclinic gallium oxide (beta-Ga2O3) single crystals have a Raman mode at similar to 250 cm(-1) that is strongly correlated with free-electron density. Prior work attributed this peak to an electronic excitation of a shallow donor impurity band. However, heavily n-type thin films grown by metalorganic chemical vapor deposition or molecular beam epitaxy do not have the peak. In the present work, an alternate model is proposed: the 250 cm(-1) Raman peak arises from Ga clusters, defined as two or more Ga atoms that form Ga-Ga bonds. Raman mapping reveals variations in the frequency that are consistent with a distribution of cluster sizes. The intensity of the peak decreases as the temperature is raised, attributed to melting of the Ga clusters. First-principles calculations indicate that the 250 cm(-1) mode is due to Ga-Ga bond-stretching vibrations. As the Fermi energy is raised, the formation of Ga-Ga dimers becomes energetically favorable, explaining the correlation between n-type conductivity and the appearance of the Raman peak.

Semi-insulating manganese-doped gallium nitride (GaN:Mn) layers epitaxially grown on unintentionally doped GaN substrates were used as photoconductors in optically addressable light valves (OALVs) to withstand higher operational laser fluences compared to current state-of-the-art OALVs where bismuth silicon oxide (BSO; Bi12SiO20) layers are used as photoconductors. GaN:Mn promises to be an exciting material for optoelectronic operations due to its large laser fluence handling capability and photoresponsivity near the band edge. The laser damage thresholds for the semi-insulating epitaxial GaN:Mn layer and the n-type substrate layer were measured to be 2.4 and 4.2 J/cm(2), respectively. These are 6-10 times higher than that of BSO (0.4 J/cm(2)). These measurements were performed by exposing similar to 200 sites on the samples to increasing fluence levels from a Gaussian pulsed Nd:YAG laser system (1064 nm) operating at a 5 Hz repetition rate with a 3 ns pulse width. Photoresponsivity of the GaN:Mn material was investigated at discrete wavelengths of 447, 405, and 380 nm. The peak photoresponsivity was observed under an illumination wavelength of 380 nm and is attributed to stronger absorption. The OALV was fabricated by attaching a 110-mu m-thick GaN:Mn layer grown on a 280-mu m-thick n-GaN layer to a 3-mm-thick BK7 optical window. A twisted nematic E7 liquid crystal was introduced to the 5 mu m gap between the two components. Transmission levels of >90% were achieved for the fabricated OALVs for a peak voltage of 40 V, constrained by transmission "bleed-through".

Due to the high fluences required for achieving ignition at the National Ignition Facility (NIF), a handful of damage sites can be initiated per UV optic, requiring them to be removed for offline mitigation and repair. As part of the NIF sustainment effort, we are developing a new programmable spatial shaper (PSS) to be installed on each beam line on NIF to carve shadows in the beam profile, blocking light at the damage spots and reducing the exchange rate of optics. At the core of the PSS is an optically addressable light valve (OALV) composed of a liquid crystal layer controlled by a photoconductor transparent at 1053 nm (1 omega) and addressed by a blue beam imprinted with the blocker pattern. We build on recent breakthroughs in wide bandgap semiconductors to qualify new photoconductor materials with sufficiently high 1 omega fluence tolerance and responsivity at blue wavelengths with aperture size which can handle the fluences and beam sizes in NIF downstream of the one-by-four splitter.

beta-Ga2O3 is an emerging ultra-wide bandgap semiconductor with great promise for power electronics and optoelectronics. Alloys in the In2O3-Ga2O3 system are interesting for optoelectronic applications, particularly where bandgap tuning is desirable. Herein, beta-(InxGa1-x)(2)O-3 alloys with target compositions x = 0.025 or 0.10 are grown from the melt using the Czochralski and vertical gradient freeze techniques. Growth with 10 mol% In yields only small, needle-like crystals, while 2.5 mol% In allows growth of centimeter-sized single crystals. A substantial degree of indium segregation is unveiled by spatial measurements of lattice parameters and the bandgap. The bandgap decreases by a maximum of 0.28 eV in the case of the highest In content crystals. Z-contrast transmission electron microscopy confirms a solely octahedral coordination of In in the beta-Ga2O3 lattice. With indium concentrations higher than 2.5 mol%, samples contain micron-scale voids that impart a dark coloration. All measured crystals are electrically conductive, with carrier concentrations varying 10(16)-10(17) cm(-3) depending upon the location of the sample in the growth. Lastly, a unique luminescence with unknown origin centered around 2.0 eV is revealed by photoluminescence spectroscopy.

Several acceptor dopants have been explored in β-Ga2O3 to produce semi-insulating substrates and epitaxial films. Fe and Mg make up the majority of research thus far; however, other transition metals provide potential alternatives for optimized performance. β-Ga2O3 bulk single crystals were grown by the Czochralski and vertical gradient freeze methods with a nominal dopant concentration of 0.25 at. % Mn. Ultraviolet-visible-near infrared spectroscopy and photoluminescence revealed polarization- and orientation-dependent optical absorptions (pleochroism) coupled with an orange luminescence. All samples were electrically insulating, on the order of 109–1011 ohm cm at room temperature, indicative of acceptor doping. Actual dopant concentrations of the intentionally doped transition metal and background impurities were determined via glow discharge mass spectrometry, indicating the macroscale segregation behavior. High-temperature resistivity measurements indicated an experimental acceptor level of 1.7 ± 0.2 eV. Hydrogenation of samples resulted in an increase in the orange luminescence and O–H stretching modes observable in the infrared spectrum. Density functional theory calculations were performed to determine the likely site-occupancy and acceptor level of Mn in the bandgap.

The semi-insulating single crystal β-Ga2O3 is becoming increasingly useful as a substrate for device fabrication. Fe doping is a method for producing such substrates. Along with Fe dopants, β-Ga2O3:Fe also contains Cr3+. Photoluminescence (PL) emission peaks at 690 nm (1.80 eV) and 696 nm (1.78 eV), as well as a broad feature around 709 nm (1.75 eV), are observed in β-Ga2O3:Fe. PL mapping of the 690 nm emission showed high and low intensity bands due to impurity striations introduced during crystal growth. PL mapping also revealed surface defects showing broad emissions around 983 nm (1.26 eV) and 886 nm (1.40 eV) that were spatially localized, occurring at discrete spots on the sample surface. Raman mapping of an 886 nm emission center revealed peaks at 2878 and 2930 cm−1, consistent with an organometallic or hydrocarbon compound. Raman mapping of the 983 nm center showed a peak at 2892 cm−1. Bright UV emission centers showed Raman peaks at 2910 and 2968 cm−1, which are attributed to Si–CH3 groups that may originate from silica polishing compounds or annealing in a silica ampoule.

Point defects have a strong influence on the physical properties of materials, often dominating the electronic and optical behavior in semiconductors and insulators. The simulation and analysis of point defects is, therefore, crucial for understanding the growth and operation of materials, especially for optoelectronics applications. In this work, we present a general-purpose Python framework for the analysis of point defects in crystalline materials as well as a generalized workflow for their treatment with high-throughput simulations. The distinguishing feature of our approach is an emphasis on a unique, unit cell, structure-only, definition of point defects which decouples the defect definition, and the specific supercell representation used to simulate the defect. This allows the results of first-principles calculations to be aggregated into a database without extensive provenance information and is a crucial step in building a persistent database of point defects that can grow over time, a key component toward realizing the idea of a “defect genome” that can yield more complex relationships governing the behavior of defects in materials. We demonstrate several examples of the approach for three technologically relevant materials and highlight current pitfalls that must be considered when employing these methodologies as well as their potential solutions.

AbstractOptically addressable light valves (OALVs) are specialized optical components utilized for spatial beam shaping in various laser‐based applications, including optics damage mitigation, and enhanced functionality in diode‐based additive manufacturing requiring high intensities. Current state‐of‐the‐art OALVs employ photoconductors such as Bismuth Silicon Oxide (BSO) or Bismuth Germanium Oxide (BGO), which suffer from limited laser‐induced damage thresholds (LiDT) and inadequate thermal conductivities, thus restricting their use in high peak and average power applications. Aluminum nitride (AlN), an emerging ultra‐wide band gap (UWBG) III–V semiconductor, offers promising optoelectronic properties and superior thermal conductivity (>300 Wm−1K−1 at 298° K, compared to BSO's 3.29 Wm−1K−1). In this study, the first AlN‐based OALVs are designed, fabricated, and experimentally demonstrated using commercially available single‐crystal AlN substrates. These AlN‐based OALVs have shown clear superiority over BSO and BGO‐based devices. Design considerations for OALVs incorporating UWBG photoconductors are discussed, and the photoresponsivity from defect‐mediated sub‐bandgap absorption in AlN crystals is verified as sufficient for OALVs operating under high light fluences. The optimum driving voltage for the AlN‐based OALV is determined to be ≈ 45 Vpp at 100 Hz, achieving a transmittance of 91.3%, an extinction ratio (ER) of more than 100, and a 51:1 image contrast.

The effects of a pulsed, focused, deep-UV (4.66 eV) laser on wide and ultra-wide bandgap semiconductors were investigated with photoluminescence (PL) and Raman spectroscopy. Three semiconductor single crystals were studied: silicon carbide (6H-SiC), gallium nitride (GaN), and gallium oxide ((3-Ga2O3). Atomic emission lines from neutral Ga or Si were observed during the laser-damage process. For all three semiconductors, PL mapping (3.49 eV laser excitation) of the damaged material revealed visible emission bands in the 2.6-2.8 eV range, attributed to point defects. Raman spectra (2.33 eV excitation) showed a reduction in the Raman peak intensities in the damaged region, along with weak PL bands around 1.9-2.1 eV. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Wide band gap and ultra wide band gap photoconductive semiconductors are of increasing interest for a number of applications including high power switching, high speed electronics, electromagnetic isolation, power electronics, and optical beam control. We will present work characterizing a number of semi-insulating semiconductors including GaN, SiC, Diamond, AlN, Ga 2 O 3 , their relative performance metrics, and examples of realized and proposed devices and applications. We will further discuss selection criteria based on first principles analysis for promising new photoconductors.

We present iron-doped beta gallium oxide (Fe- $\beta $ Ga $_{{\text{2}}}$ O $_{{\text{3}}}$ ) as a candidate for photoconductive semiconductor switches (PCSSs) with sub-bandgap light. From a commercially available Fe- $\beta $ Ga $_{{\text{2}}}$ O $_{{\text{3}}}$ wafer, we first did material characterization. This included measurements of absorption coefficient and dopant composition, carrier activation energy up to 200 $^{\circ}$ C, break down field of planar electrodes (limited from material passivation), and free carrier recombination lifetime, and thermal effects up to 203 $^{\circ}$ C on photocurrent with a 447 nm light emitting diode (LED) source. We then demonstrated pulsed operation of a Fe- $\beta $ Ga $_{\text{2}}$ O $_{\text{3}}$ PCSS under different sub-bandgap wavelengths (355, 532, and 1064 nm) and sub-ns pulses. Fe- $\beta $ Ga $_{\text{2}}$ O $_{\text{3}}$ is a candidate for high temperature PCSS with 355 nm responsivity of 7 $\times$ 10 $^{-\text{7}}$ A-cm/W-kV at room temperature and up to 5.5 $\times$ 10 $^{-\text{4}}$ A-cm/W-kV at 200 $^{\circ}$ C. From these investigations, we discuss a simple trap model to describe the illumination process of the PCSS. Fe- $\beta $ Ga $_{\text{2}}$ O $_{\text{3}}$ has a high breakdown field and has moderate responsivity characteristics, but the dark current at high temperature leads to low photo-to-dark current ratio (PDCR). Regardless, we verify its potential as a PCSS material for harsh environment applications.

This article provides an overview of power semiconductor devices (PSDs) for the distributed energy resource (DER) system.To begin with, an overview of electrically triggered silicon carbide (SiC) and gallium nitride (GaN) devices followed by a brief narration of ultrawide bandgap (UWBG) PSDs and, subsequently, an overview of optically activated PSDs encompassing photoconductive semiconductor switch (PCSS) and optical bipolar PSDs are provided.Finally, an overview of PSD packaging and reliability is captured.

In this work, we demonstrate and model the deep-level defect physics of semi-insulating gallium arsenide bulk photoconductive semiconductor switches (PCSS) with gap size of $10 ~\mu \text{m}$ and $25 ~\mu \text{m}$ in dark-mode operation. Experimental measurements up to biasing field of 10 kV/cm show near-bistable characteristics in the dark-mode current-voltage relations for the PCSS, which cannot be reproduced through commercial Technology Computer-Aided Design simulations. Thus, we model the PCSS by solving for homogeneous non-equilibrium steady-state of the PCSS trap dynamics, where we introduce two semi-analytical models both involving two deep levels with impact ionization effects. Both models have an excited deep-level that can capture electrons from or emit electrons to the conduction band. The two models differ, however, by the fact that one has a ground state with capture and emission, whereas the other does not include such mechanisms but instead includes electron excitation and relaxation processes directly between the ground state and the excited state without interactions with the conduction band. We find that the former does not fit with experimental near-bistable features while the latter achieves a good match with the same total number of fitting parameters. Further measurements of bias upto 50 kV/cm on one $10 ~\mu \text{m}$ PCSS confirms the validity of the second model as well. Finally, a brief discussion of the implications on the illuminated operation of the PCSS is also given to illustrate the importance of including defect interactions and defect avalanche effects.